Use dedicated dts folder like on ramips to store device tree source
files for boards not already supported in vanilla Linux.
Doing so instead of having them in files-* has several advantages:
* we don't need to duplicate them for several kernel versions
* changes to a device tree don't trigger a complete kernel rebuild
* the files are more obvious to find
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
